Thermal spray composition and method of deposition for abradable seals

ABSTRACT

A thermal spray composition and method of deposition for abradable seals for use in gas turbine engines, turbochargers and steam turbines. The thermal spray composition comprises a mixture of metal-clad solid lubricant particles and unclad solid lubricants particles for producing an abradable seal used in the compressor section of gas engines, aircraft engines, radial compressors and the like. The metal is selected from alloys of Ni, Co, Cu, Fe and Al, preferably Ni alloys, and the solid lubricant is at least one of hexagonal boron nitride, graphite, calcium fluoride, lithium fluoride and molybdenum disulphide, preferably hexagonal boron nitride or hexagonal boron nitride and graphite.

This application is a Divisional of application Ser. No. 10/346,124filed Jan. 17, 2003 now U.S. Pat. No. 6,808,756.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

This invention relates to abradable seals and, more particularly,relates to high-temperature abradable seal compositions for use in gasturbine engines, turbochargers, compressors, steam turbines and thelike.

(ii) Description of the Related Art

Basic requirements for abradable seals in the compressor section of gasturbine engines include good abradability, spall resistance, and erosionresistance. Abradable seals are also required to exhibit low gaspermeability, a smooth surface, good aging properties and long-termoxidation resistance at high temperatures. In the case of abradability,the seal is a sacrificial element, it being desirable to minimize bladewear. Additionally, low gas permeability is required in order tominimize gas flow through the seal itself. It has been shown that lowpermeable seals with a smooth surface finish improve overall compressorefficiency by about one percent as compared to conventional permeableseals. In addition, low permeability of the seal prevents entrapment offine particles, e.g. dust or grit, which can act as an abrasive againstthe blade tips, thus wearing them unevenly. Smooth surface finishes inthe gas path improve overall airflow, also contributing to efficiency.Finally, long-term oxidation resistance is required due to increases incompressor operating temperature up to 815° C.

There are several air seals used in a compressor section of a gas oraircraft engine. Historically the oldest is feltmetal that comprises aplurality of metal fibres. The feltmetal is described for example inU.S. Pat. No. 4,257,735. The most important disadvantages of this sealare that it has to be brazed to the substrate material and that it ishighly porous.

Typical jet engine compressor air seals include a metal matrix ofaluminum-silicon with embedded polymer particles or hexagonal boronnitride powder particles as described in U.S. Pat. No. 3,723,165 andU.S. Pat. No. 5,506,055, respectively. The disadvantage of these systemsis their limited temperature capability at 315° C. for the system withpolymer and 480° C. for the system with hexagonal boron nitride. In theformer case, the temperature capability is governed by the polymer andin the latter case it is governed by the aluminum silicon alloy.

Abradable materials used at high temperatures in the compressor sectionof turbine engines are usually NiCrAl/Bentonite coatings described inU.S. Pat. Nos. 4,374,173 and 4,291,089 by Adamovic. However,NiCrAl/Bentonite seals do not rub well against Ti alloy blades. Thesecoatings perform well against Ni alloy and steel blades but, when Tialloy blades are used, the blade tips overheat and are subject to wear.Sometimes, glazing of the coating is observed.

Another known abradable seal is that prepared by the techniques ofRangaswamy et al., described in U.S. Pat. No. 5,434,210. A compositepowder for thermal spraying of abradable coatings is disclosed in whichthe composite powder contains three components. One component is any ofa number of metal or ceramic matrix materials, another component is asolid lubricant, and the third component is a polymer. Typicalas-sprayed coatings comprise a Co alloy matrix with dispersed particlesof hexagonal boron nitride and polymer. The polymer is subsequentlyburned out and the final very porous structure contains only hexagonalboron nitride particles dispersed throughout the Co-based matrix. Thecoatings prepared from this material have acceptable abradability butlow erosion resistance. The erosion resistance is required in order tomaintain uniform clearances throughout the life of the engine or engineperformance characteristics are adversely affected. Conventionalcommercial turbine engines have exhibited a two percent increase inairflow around blade tips as a result of seal erosion afterapproximately 3,000 flights. Much of this may be attributed to erosionof the abradable seal and blade airfoil tip, and to rub interactionsbetween the blade tips and the seal. In military engine applications,where gas path velocities are relatively high, erosion resistance is ofparamount importance.

Moreover, high permeability due to open porosity of conventional sealsenables back leakage of gas, which decreases engine efficiency.

We have found that the use of Ni alloy-clad hexagonal boron nitridepowder applied by flame or plasma spraying provided an abradable sealwhich exhibited poor combination of erosion resistance and abradability.When erosion resistance was acceptable, the abradability was poor. Whenabradability was satisfactory, the erosion resistance was poor. Nialloy-clad boron nitride, accordingly, is not suitable for use as anabradable seal.

It is accordingly a principal object of the present invention to providea novel thermal spray composition and its method of application forproducing an abradable seal.

A further object of the invention is the provision of an abradable seal,for use in gas turbine engines having good abradability, spallresistance and erosion resistance, particularly when used in conjunctionwith titanium-alloy blades.

It is another object of the present invention to provide an abradableseal having a smooth surface, low permeability, good thermalconductivity, low interparticle cohesive strength and long-termoxidation and glazing resistance resulting in favourable long-term agingcharacteristics.

SUMMARY OF THE INVENTION

In its broad aspect, the thermal spray powder composition of theinvention for an abradable seal comprises a mixture of metal cladlubricant powder and unclad lubricant powder, said mixture having about5 to 30 wt %, preferably about 10 to 20 wt %, unclad lubricant powder,and about 70 to 95 wt %, preferably about 80 to 90 wt %, of a metal-cladsolid lubricant powder having a size in the range of 10 to 150 microns.The composition can be in the form of a powder or a powder consolidatedas a wire or rod. The solid lubricant is at least one of hexagonal boronnitride, graphite, calcium fluoride, lithium fluoride and molybdenumdisulphide particles, preferably hexagonal boron nitride powder or amixture of hexagonal boron nitride and graphite. The matrix-formingmetal, a metal alloy cladding, is selected from Ni, Co, Cu, Fe, Al, andcombinations and alloys thereof, particularly nickel alloys such asNiCrAl and NiCr, and CuAl and AlSi.

The matrix-forming metal alloy and the solid lubricant can also containsome other elements as impurities, that significantly do not alter theseal properties.

The composition can also contain a fugitive phase to create porosityafter the elimination from the abradable coating, the fugitive phaseconsisting at least one of salt, sugar and other fugitive materials.

In accordance with another broad aspect of the invention, thecomposition comprises about 70 to 95 wt % of a metal alloy-clad solidlubricant and about 5 to 30 wt % of unclad solid lubricant.

The method of providing an abradable seal on a substrate comprisesapplying an adherent coating of the said powder composition having athickness of up to 3 mm to the substrate by thermally spraying thepowder composition thereon such as by plasma spraying, combustionspraying or wire spraying.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The abradable seal of the present invention comprises a matrix-formingmetal or metal alloy component and a solid lubricant component, whereinthe two components provide a synergism in abradable coatings which haveunexpected superior characteristics over prior art materials. Thematrix-forming metal alloy present as a metal cladding on a solidlubricant powder is selected from the metals Ni, Co, Cu, Fe, Al andcombinations and alloys thereof, preferably NiCrAl, NiCr, CuAl and AlSi.The solid lubricant is typified by lubricants such as hexagonal boronnitride, graphite, calcium fluoride, lithium fluoride and molybdenumdisulphide particles, preferably hexagonal boron nitride or hexagonalboron nitride and graphite particles. A preferred composition of metalalloy clad on a solid lubricant is hexagonal boron nitride or hexagonalboron nitride and graphite particles clad with NiCrAl alloy. Themetal-clad solid lubricant particles are blended with unclad particlesof solid lubricant particles such as hexagonal boron nitride orhexagonal boron nitride and graphite available from commercialsuppliers.

The unclad solid lubricant component comprises at least 5 wt % and notmore than 30 wt % of the composition, preferably 10 to 20 wt % of thecomposition, the balance comprising the metal clad solid lubricantcomponent. The metal-clad solid lubricant component comprises at least60 wt % and not more than 95 wt % of a metal phase and at least 5 wt %and not more than 40 wt % of a solid lubricant phase.

Although the description proceeds herein with reference to a blend ofmetal-clad solid lubricant particles and unclad solid lubricantparticles, it will be understood that the powder composition may also beprepared in the form of agglomerated particles or in the form of wire orrod.

The two-phase powder composition of the invention preferably is appliedto a substrate by thermal spray to form an abradable seal. Thermalspraying involves the softening or melting of a heat fusible metalcomponent material by heat and propelling the softened or meltedmaterial in particulate form against a surface to be coated. The heatedparticles strike the surface where they are cooled and bonded thereto.The solid lubricant particles impact the coating surface and adhere toit mainly by mechanical interlocking. Usually the percentage of solidlubricant in the coating is less than in the powder because themetal-clad particle sticks better to the target than the solid lubricantparticle. A conventional thermal spray gun may be used for the purposeof the both heating and propelling the particles.

A thermal spray gun normally utilizes a combustion or plasma or electricarc to produce the heat for melting of the powder particles. In a powdertype combustion thermal spray gun, the carrier gas, which entrains andtransports the powder, is typically an inert gas such as argon. In aplasma spray gun, the primary plasma gas is generally argon or nitrogen.Hydrogen or helium is usually added to the primary plasma gas, and thecarrier gas is generally the same as the primary plasma gas. Otherthermal spray methods could also be used. A good general description ofthermal spraying is provided in U.S. Pat. No. 5,049,450.

The matrix-forming metal-clad solid lubricant powder such as NiCrAl-cladhexagonal boron nitride can be dry blended and mixed with the uncladpowder such as hexagonal boron nitride. The matrix metal alloy holds theparticles of the solid lubricant in place and adherently bonds thecoating to the substrate.

The solid lubricant particles are uniformly dispersed throughout thedeposited coating. Thus any material removal that occurs does not changecoating properties of the remaining seal. Coatings of the inventionprepared by thermal spraying the coating composition onto a substratehave an excellent combination of abradability and erosion resistance asa result of the coating properties including low coating cohesivestrength, low porosity, low surface roughness and high thermalconductivity. As a result, the coatings can be effectively used incombination with titanium-alloy blades without danger of titanium fire,coating glazing or excessive blade wear. Overall engine efficiency isincreased.

The powder composition is typically prepared as a blend. The metalliccontent of the metal-clad solid lubricant component provides coatingmatrix that holds the solid lubricant particles in place, bonds thecoating to the substrate and provides oxidation resistance. An additionof the unclad solid lubricant powder component causes a decrease in thehardness and strength of the metallic matrix material owing toseparation of metallic particles by solid lubricant particles andenhances the ejection of the particles from the coating when abraded.Due to its softness it decreases blade wear and improves abradability.The clad solid lubricant also contributes to the decrease in the matrixstrength and hence improves abradability. During the rubbing process byengine blades, the material abrades readily due to low matrix strengthcaused by the effect of the solid lubricant particles. The metal alloymatrix would collapse and compress during a rubbing process without thesolid lubricant filler and the coating would become very hard withsubsequent significantly decreased abradability. Coating glazing islimited by easy particle ejection when rubbed by a compressor blade.Coatings prepared by thermal spraying this powder have excellentabradability and negligible blade wear under a variety of rubbingconditions. These properties directly follow from low coating cohesivestrength, low porosity, low surface roughness and high thermalconductivity. As a result the coatings could be used against Ti alloyblades without danger of Ti fire, coating glazing and excessive bladewear throughout the full range of temperatures at which titanium bladesare used (up to 600° C.) and above where nickel and steel based bladesare used.

The present invention provides a number of important advantages. Theaddition of unclad solid lubricant increases abradability and decreasesblade wear, without sacrificing erosion resistance. Glazing due tosevere blade rubbing is minimized, which can be attributed to the lowcoating interparticle strength and good thermal conductivity. It hasbeen found that the enhanced thermal conductivity due to the densestructure promotes dissipation of heat generated during severe rubconditions, thereby decreasing the chance of titanium fire.

It will be understood, of course, that modifications can be made in theembodiments of the invention illustrated and described herein withoutdeparting from the scope and purview of the invention as defined by theappended claims.

1. A thermal spray powder composition for an abradable seal comprising about 70 to 95 wt % of a metal-clad solid lubricant and about 5 to 30 wt % solid lubricant.
 2. A thermal spray powder composition for an abradable seal comprising about 80 to 90 wt % of a metal-clad solid lubricant and about 10 to 20 wt % of unclad solid lubricant.
 3. A thermal spray composition as claimed in claim 1, in which the solid lubricant is at least one lubricant powder selected from the group consisting of hexagonal boron nitride, graphite, calcium fluoride, lithium fluoride, magnesium fluoride, barium fluoride, tungsten disulfide and molybdenum disulfide particles.
 4. A thermal spray composition as claimed in claim 1, in which the solid lubricant is a mixture of hexagonal boron nitride and graphite particles.
 5. A thermal spray composition as claimed in claim 1, in which the solid lubricant is hexagonal boron nitride particles.
 6. A thermal spray composition as claimed in claim 3, in which the thermal spray composition is a powder, a wire or a rod and in which the metal alloy cladding is selected from the group consisting of Ni, Co, Cu, Fe, Al, and combinations and alloys thereof.
 7. A thermal spray composition as claimed in claim 4, in which the metal alloy cladding is selected from the group consisting of Ni, Co, Fe and combinations and alloys thereof.
 8. A thermal spray composition as claimed in claim 5, in which the metal alloy cladding is Ni, NiCr, NiCrAl or Ni alloy.
 9. A thermal spray composition as claimed in claim 5, in which the metal alloy cladding is NiCrAl.
 10. A thermal spray composition as claimed in claim 5, in which the metal alloy cladding is selected from the group consisting of Ni, Co, Fe and combinations and alloys thereof.
 11. An abradable seal for gas turbine engines, turbo changers, compressors or steam turbines produced by applying an adherent coating having a thickness up to 3 mm to a substrate by thermally spraying a composition thereon comprising about 70 to 95 wt % of a metal-clad solid lubricant comprised of hexagonal boron nitride clad with a metal selected from the group consisting of Ni, NiCr, NiCrAl and Ni alloy, and about 5 to 30 wt % of a solid lubricant comprised of hexagonal boron nitride. 